Mercapto acids are generally esterified by heating mercapto acids in a reaction mixture containing an alcohol, a strong acid catalyst, and optionally, a solvent. ##STR1## If excess alcohol (.about.3 mole equivalents) is used, this reaction will usually yield approximately 95% ester in a crude reaction mixture with the starting materials and water (which is formed as a byproduct). Unfortunately, only a 65% yield of ester can be purified from this crude reaction mixture by conventional means such as distillation. The 30% loss of yield of ester appears to be the result of hydrolysis which occurs during the distillation process.
As the distillation is performed on the crude reaction mixture the first material removed is the excess short chain alcohol. Removing the short chain alcohol shifts the equilibrium of the reaction towards favoring the hydrolysis of the ester back to the mercapto acid and alcohol starting materials. If a method of removing the water from the crude reaction mixture prior to distillation could be discovered, the hydrolysis of the ester could be avoided.
Various methods have been tried to remove the water from the crude reaction mixture set forth above, such as the addition of drying agents or the extraction of water utilizing simple organic solvents. Neither of these methods has proven to be satisfactory. On a commercial scale it is economically unfeasible to use conventional drying agents to remove water. Extracting the water utilizing simple organic solvents such as toluene or chloroform has also been tried but with limited success. The simple solvent systems either leave too much water in the organic phase (which results in hydrolysis of the ester) or leave too much of the ester produced in the aqueous phase which is to be discarded.
Thus, it would be a significant contribution to the art to develop an alternative purification process for recovering substantially water free esters of mercapto acids.
Additionally, it would be particularly advantageous to provide a process for recovering esters of mercapto acids from a reaction mixture containing water which allows for recovery of substantially all of the ester present in said reaction mixture.
It would also be advantageous if a purification process for recovering esters of mercapto acids could be developed which economically provided for the recovery of a substantially water free ester.
It is an object of this invention to provide an alternative purification process for the recovery of substantially water-free esters of mercapto acids.
It is also an object of this invention to provide a process for recovering substantially all of the ester of a mercapto acid present from a reaction mixture containing water.
It is a further object of this invention to provide an economical process for recovering a substantially water free ester of a mercapto acid.
Other aspects, objects, and several advantages of this invention will be apparent from this specification.